1. Field of the Invention
The object of the invention is a process and installation for planishing under tension a thin metal strip.
2. Background Information
Hard, thin, sheet steel such as tinned sheet iron, used notably in the manufacture of packaging, is produced by rolling in the form of long lengths of thin metal strip which is subsequently treated, split and shaped according to the intended use.
In general, a thin metal strip must exhibit a certain number of qualities such as excellent flatness and an aptitude for stamping, and surface state and mechanical properties complying with the standard specifications corresponding to the desired applications.
To obtain these qualities, the metal strip undergoes a certain number of processing stages and is, in particular, subjected to planishing which is often performed by stretcher-and-roller leveling with prescribed elongation.
A planishing machine generally comprises one or two bending units each made up of a pair of small-diameter rolls placed on both sides of the strip and which are offset in height so as to set up, by their imbrication, an elbowed path for the strip producing reverse bendings on the two rolls.
The strip is brought under tension between the two tension blocks placed in position on both sides of the machine, each one comprising several imbricated rolls on which the strip travels. The rolls making up the two tension blocks are driven in rotation at slightly different speeds such that the through-speed of the strip in the downside tension block is fractionally higher than its through-speed in the upside block.
This results in permanent elongation of the strip, the value of which can be determined by adjusting the difference between the upside and downside through-speeds.
Planishing is performed by subjecting the strip while under tension to at least two reverse bendings on small-diameter rolls. It is known that, at the time of each bending, the external stretched part of the strip can be in the domain of plastic deformation, even if the tensile stress applied is well below the elastic limit. It is therefore possible, without subjecting the strip to excessive stress, to induce, between the two tension blocks, sufficient elongation to exceed the length of the longest fiber so as to planish the strip by equalizing the lengths of all its longitudinal fibers.
The smaller the radii of curvature and the greater the tensile stresses, the more substantial the permanent elongation values for a given product become, the tensile stresses nevertheless always remain below the elastic limit.
It is thus possible to obtain smooth strips, but these operations induce internal stresses within the thickness of the strip. These balance out, but nevertheless result in the deformation of the strip's profile, which generally presents a camber in the transversal direction, and quite often, a camber in the longitudinal direction too.
Transversal camber can be relatively easily corrected on a transversal camber correction device placed in position on the downside of the planisher. This device comprises a roll that bears against the strip on the side opposite that on which the last roll of the planisher bears and is associated with two larger-diameter rollers.
The longitudinal camber fault can be corrected on a longitudinal camber correction roll although in the case of very thin, hard strip, this correction is difficult to perform, particularly due to the very high sensitivity of the devices used. Indeed, for very thin strip, faults are very transitory and fluctuating for a given machine adjustment.
Yet residual longitudinal camber can impede the introduction of the strip into the succeeding processing installations and can, in addition, result in deformations when the strip is cut into narrow bands according to the dimensions of the products to be produced.
While it is possible to manually adjust the machine's settings, the high winding speed of the strip only allows correction of detectable faults and, unavoidably, only after a certain delay. Furthermore, the settings might have to be readjusted again when the next coil begins feeding.